1. Field of the Invention
This invention relates to heading sensors, and more particularly to the calibration of heading sensors.
2. Description of the Related Art
Heading sensors are used in a variety of applications such as personal handheld devices, automobiles, airplanes and boats. Heading sensors typically indicate a heading relative to true North although other references are also possible. The heading is typically specified as an azimuth angle from true North. Multi-dimensional sensors may also provide a pitch measurement. Pitch is typically specified as an elevation angle from the local level.
Heading sensors include magnetic compasses that sense the Earth's magnetic field and inertial compasses that exploit the Earth's rotation. A simple magnetic compass includes a magnetized pointer usually marked on the North end free to align itself with the Earth's magnetic field. A compensation, typically in the form of a declination table, is required to find true North. More sophisticated magnetic compasses may include multiple magnetometers positioned orthogonal to each other to provide full 3-axis measurements. A digital magnetic compass may include three magnetometers to measure magnetic fields in three axis and three accelerometers to measure and compensate for the tilt angle of the device. A simple inertial compass is essentially a gyroscope, a spinning wheel mounted on gimbals so that the wheel's axis is free to orient itself in any way. Torque is applied to the axis of the gyroscope causing the axis to process towards the North celestial pole. More sophisticated inertial compasses include multiple orthogonal gyroscopes to provide full 3-axis measurements. An inertial measurement unit (IMU) incorporates sensors to allow for measurement of translation and rotation in each of three axes. Typically the IMU is implemented with three accelerometers and three gyroscopes. The accelerometers are placed such that their measuring axes are orthogonal to each other. They measure inertial acceleration, also known as G-forces. Three gyroscopes are placed in a similar orthogonal pattern, measuring rotational position in reference to an arbitrarily chosen coordinate system. IMUs are typically used as part of inertial guidance systems to maneuver aircraft. Hybrid magnetic/inertial compasses that use a configuration of magnetometers and gyroscopes may be used. These more sophisticated compasses may provide more accurate heading measurements, pitch measurements and tilt compensation.
Heading sensors may be calibrated to ensure the sensor provides accurate measurements. Magnetic compasses may be calibrated to compensate for declination (the difference between magnetic north and true north), magnetic tilt or inclination (local deviation in the magnetic vector from magnetic north) and local magnetic fields (nearby magnetic objects) that distort the measurements. Declination is a function of geographic position and may be corrected by a constant correction factor for a particular geographic area. Tilt is a function of the roll and pitch of the compass. If the compass is provided with the necessary gravitational acceleration sensors tilt can be measured and largely removed via calibration. Local magnetic fields are a function of the local environment and in some cases can be removed via calibration. Typically, a sensor-specific procedure is provided to remove the local magnetic fields. Inertial sensors suffer from drift of the gyroscopes over time. Both magnetic and inertial sensors are characterized by measurement noise that cannot be removed.